An underground rock drill rig generally has at least one boom on which is mounted a feed rail. The feed rail supports a drill and its associated drill rod.
The drill is mounted to the rail, so as to be able to move along the length of the rail. When the drill is in an initial position at the rear of the rail, the drill rod is supported at the front of the rail by a bush, which is held in place by a centralising clamp.
Centralising clamps are generally formed from two pieces, each of which has a semi-circular recess which can locate about one side of the bush. The two pieces can be clamped together using a nut and bolt in order to hold the bush in place in between the two pieces.
There are some inherent weaknesses in this arrangement. The entire clamping force acts through the bolt, providing significant stress concentrations about the bolt. Away from the line of the bolt, the clamping forces are significantly dissipated, meaning that there can be a propensity for the bush to move or vibrate. Additionally, vibration during use can cause loosening of the nut, exacerbating the problem. Movement of the bush causes a significant increase in the wear rate of the centralising clamp, as well as a loss of accuracy in drilling.
As the two pieces must be able to move relative to each other, it is only possible to weld one of the pieces in position. This limits stability of the arrangement, and can lead to cracking and fatigue failure.
A further limitation of the known arrangement is that the bush is located at a designated height about the rail, being the same height as the drill. Should the rail bend, the location of the bush can not be adjusted. When this happens, it is necessary to cut away the centraliser clamp and reposition at a new location. This can cause significant down-time for the drill rig.
The present invention seeks to provide an alternative method of clamping a centralising bush in position.